The invention relates to a new type of insulating pieces for use in particular in electrical, heat and sound insulation, constituted by a stack of superposed layers of fibers capable of withstanding high temperatures. The invention also relates to a manufacturing process.
A number of processes for manufacturing insulating pieces in different shapes are known already.
French Patent FR-A-No. 1 213 048, describes such a process which consists in placing over a suitable form, a wet layer of cellulosic fibers, and in compressing said layer by means of a strip of elastic cloth, so as to exude the excess of water, and in finally drying the piece. This technique which is known to give excellent results in electrical applications, has nevertheless a number of drawbacks. Indeed, the pieces, being made with cellulosic fibers, cannot work at temperatures higher than 90.degree.-100.degree. C. Moreover, the form must allow for the shrinkage of the fibers. And finally, to produce a thick product, it is necessary to proceed in successive stages, with intermediate drying between each stage, which is lengthy and expensive and necessarily leads to a certain heterogeneity in the finished product.
Several other methods have already been proposed for producing pieces designed to work at high temperatures, namely temperatures above 800.degree. C. For example, it has been proposed to use aqueous suspensions of refractory fibers, such as ceramic fibers, and to deposit said fibers by suction through a perforated form. This technique has undeniable advantages such as for example the homogeneity of the produced pieces, throughout their thickness. Also this technique is perfectly adapted to mass production of pieces of simple shapes, readily removable for the mold. But this technique is not economically advantageous for the production of unitary pieces. It even becomes quite unusable for producing complicated unmoldable shapes. And further, there is a practical limit to the thickness of the parts which can be produced, limit which is situated around 100 millimeters, whereas the normal thickness required to give an efficient heat insulation at temperatures above 1100.degree. C., is often in excess of these limits.
According to an improved variant described in French Patent No. 1 602 362 (corresponding to British Pat. No. 1 296 681), it has been proposed to deposit over the porous form, successive layers of fibers arranged in regular order of heat resistance. As in the previous case, the water is removed from the different wet layers by suction through the form. Because precisely of that suction, as already indicated, the thickness of the finished product can hardly exceed 100 millimeters, which considerably restricts its practical aspect.
All of said processes cannot be used to obtain pieces of a density higher than 0.3, which are mechanically resistant. Yet, industries are demanding those mechanical characteristics more and more for high temperature-insulating pieces. Moreover, these processes only permit a small variation of the density and completely exclude the preparation of fluid-tight pieces. These drawbacks therefore limit considerably the scope of utilization of these pieces, particularly because of the lack of mechanical strength, of their excessive porosity and of the corrosion that may result therefrom, and of the limited thickness of the pieces.